Joint arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural joint is replaced by a prosthetic joint. Joint arthroplasty is commonly performed for hips, knees, elbows, and other joints. The health and condition of the joint to be replaced dictate the type of prosthesis necessary to replace the natural joint.
In a total hip arthroplasty an acetabular cup is implanted in the acetabular cavity of the pelvis to replace the natural acetabulum. Replacement of the acetabulum is necessary for various joint conditions, such as when there is an inadequate articulation surface for a head or ball of a prosthetic femoral component. Total hip arthroplasty is also warranted in certain cases of Developmental Dysplasia Hip (DDH) where the natural acetabular cavity does not properly form to allow sufficient joint articulation.
To implant an acetabular cup, a cavity is reamed in the acetabulum. The acetabular cup prosthesis is then inserted into the formed cavity and secured by mechanical means, interference fit, or by a combination thereof. The acetabular cup prosthesis is positioned in the pelvis at a fixed orientation with respect to patient anatomy and should remain stable.
In cases where the acetabular cavity is not generally spherical, an oblong acetabular cup may need to be implanted. Such situations include certain DDH cases and joint conditions where a previously implanted acetabular cup prosthesis (typically a hemispherical cup) has migrated in a superior direction. The oblong geometry of the cup compensates for the elongated acetabular cavity. One type of elongated acetabular cup has an outer contour defined by two similarly dimensioned adjacent hemispheres. An exemplary dual hemisphere acetabular cup 10 is shown in FIGS. 1A and 1B. The cup 1 has an outer surface 2 defined by a first primary hemisphere 4 merged with a secondary hemisphere 6. The primary hemisphere defines a primary face 8 and the secondary hemisphere defines a secondary face 9. The primary hemisphere 4 has a concave inner surface 7 that is matable with a liner which, in turn, receives a head or ball portion of a femoral hip component.
Most acetabular cup prostheses rely, at least to some extent, on an interference fit between the bone engaging surface of the implant and the cavity formed within bone. Thus, the acetabular cup, including the two hemispheres of oblong acetabular cups, tend to be oversized (by about three percent) with respect to the diameter of the cavity into which they are implanted.
As noted above, oblong or dual-hemisphere acetabular cups tend to have similarly sized lobes or hemispheres. This design can render challenging the seating of the acetabular cup prosthesis within a patient. For example, the primary hemisphere must be properly aligned and seated while the secondary hemisphere must be seated so that it does not protrude from its machined cavity. In some instances attempts to seat properly the secondary hemisphere may affect the seating and alignment of the primary hemisphere.
Most oblong or dual cups also place equal amounts of load on both of the primary and secondary hemispheres and hence the cavities into which they are placed. The secondary hemisphere may offer a lower quality of bone, or less bone, and would benefit from receiving a lesser share of the load.
Various designs of dual lobe or oblong acetabular cups are known in the art. For example, U.S. Pat. Nos. 5,192,329; 5,290,315; and 5,370,704, all of which share a common specification, describe an oblong acetabular cup in which the primary and secondary hemispheres are separate, but are joined together pre-operatively. U.S. Pat. No. 5,326,368 discloses a modular acetabular cup that includes extension member, or augments, that are attachable to the cup to enable it to achieve a cross-section of a desired configuration. U.S. Pat. No. 5,176,711 describes an acetabular cup prosthesis having an augmentation piece or defect lobe that is separately attachable to the primary acetabular cup by a Morse-taper arrangement. U.S. Pat. No. 4,892,549 describes a single acetabular cup having first and second spherical surface portions where the radius of the first spherical surface portion is slightly greater than the radius of the second spherical surface portion.
Despite the designs that now exist for oblong acetabular cups, it would be desirable to provide an oblong acetabular cup which may be seated more easily within a patient's body and which distributes more load to the anatomical cavity than to the defect cavity.